Monday, March 1, 2010

Tiny creatures with outsized influence

[Originally posted to MBA Student Oceanography Club (SOC)]

In our February meeting, we
talked a little bit about some often overlooked, but really important
creatures, including foraminifera, coccolithophores and zooxanthellae.
These tiny organisms, along with a few others, have an outsized
influence on the biology and the geology of our planet, so I thought I
would share a little more about them. To appreciate the magnitude of
their effects you have to remember that these organisms are major
constituents of the plankton, and as such fill the surface waters
covering over 70% of the earth’s surface. This means that in aggregate,
these tiny organisms play an important role in capturing sunlight,
cycling nutrients, driving the make-up of the atmosphere and creating
geological formations. For example, phytoplankton, a mix of
photosynthesizing algae and bacteria, account for as much as 50% of the
world’s primary productivity, more than rainforests, grasslands, marshes
or coral reefs. Small shell-forming organisms are also a major force
in the earth’s carbon cycle, where atmospheric carbon, such as the
greenhouse gas CO2, is absorbed by the oceans and ultimately
locked up in marine sediment and rock. I’ll focus on a few major groups
of single celled algae and animal-like protists that help make this
planet a comfortable place for us to live.

Animal-like Marine Protists

Foraminifera (often called forams for short) are
single celled protists that form shells, and can be found living on the
bottom of the sea or drifting in the plankton. Planktonic forams almost
all form calcium carbonate shells by pulling carbon out of the water.
When the forams die, these shells fall out of the water and accumulate
in huge numbers on the ocean floor where, along with coccoliths (see
below), they contribute to the formation of carbonate rocks (i.e.
limestone and chalk). This is an important process driving the earth’s
carbon cycle, and leads to the long-term storage of carbon in the
earth’s crust.

Radiolarians are similar to forams, but most form
their beautiful shells from silica. Radiolarians can be very common in
the tropics where their fallen shells cover the bottom in what is
commonly called radiolarian ooze, which can harden into siliceous rocks
(i.e. chert and flint).

Plant-like Marine Protists

Coccolithophores create calcium carbonate plates
(coccoliths) that form an armored surface, they therefore store carbon
in a manner similar to foraminifera. However, because they are able to
photosynthesize, they have an even greater effect on atmospheric carbon
by directly consuming CO2. Coccolithophores can form huge
blooms that are easily visible from space (the milky green water in the
photo shows such a bloom off the coast of Alaska), and help drive the
exchange of gases between the ocean’s surface and the atmosphere.

Diatoms form silica shells, and are probably the
most common component of the phytoplankton. As a result, diatoms are
one of the most important sources of photosynthesis on the planet; it
has been estimated that 20-30% of atmospheric oxygen is produced by
diatoms, as much as the world’s
combined tropical rainforests. This photosynthesis also takes up lots of CO2, and the heavy silica shells quickly drag this carbon to the bottom of the ocean.

Dinoflagellates are a diverse group of marine
protists, that unlike the others, do not form hard mineral shells
(though some have hard, cellulose coverings). Their inclusion among the
plant-like protists is a bit arbitrary, as about half of the
dinoflagellates do not photosynthesize, but many of the best-known
examples do. These include species that give rise to red tides and
other harmful algal blooms (like the red tide off the coast of
California pictured above), species that create the glow in
bioluminescent water, and the zooxanthellae that live symbiotically
within the tissues of a number of other organisms, including corals,
jellyfish, clams, sea slugs and even foraminifera and radiolarians.